27
3.4
Simulation Platform
We have developed a comprehensive simulation testbed to evaluate various server
designs. The simulator, written in CSIM, consists of three major components; a com 
munication layer, a kernel layer and a Web server layer. Figure 3.4 depicts the simple
diagram of the simulator. A Web server module opens TCP connection as a request
comes from the distributor. Requests are served as either dynamic or static contents. If
the backend forward mechanism is used, it calls VIA functions to send requests to the
other lightly loaded servers. For intra cluster communication, the simulator supports
the VI Provider Library (VIA communication modules) and the network interface card's
(NIC) device driver. The VIA communication modules, which follows the semantics of
the VIA specification, support most of the VIA functionalities including blocking com 
munication, non blocking communication, communication with Completion Queue (CQ),
and communication with receive and send queues. The VIA functions communicate with
the network interface card's (NIC) device driver using the doorbell mechanism for intra 
cluster communication. A NIC checks its doorbell to know if there are communication
requests from application programs. After finishing requests, it adds the communication
information to the CQ or send queue.
At the kernel layer, we model the Linux scheduler and the coscheduling modules.
The Linux scheduler in our simulator is modeled from Linux2.4.7 10. The scheduler
maintains several wait queues and a single run queue that contains all the processes
currently ready to execute. Each time the scheduler runs, it loops through the entire
run queue and selects the process with the highest goodness value. The goodness value












  

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